For many years, cross-connections and backflow have been well understood. Programs to prevent unhealthy situations from happening have been in place for many years. However, a general and extensive study on their impacts, types of programs, incidents, and financial repercussions has not been conducted. Many computer models for steady state or quasi-steady state simulation exist, but applications for transient state simulation, especially demonstrating backflow incidents, have rarely been done. The first objective of this project was to assess the impacts, financial and otherwise, of cross-connections and cross-connection control in North American water supplies and to North American water purveyors. The project team also planned to create and use a computer model to simulate and demonstrate hydraulic changes in the transient state. Of the over 700 utilities surveyed, 91 per cent were found to have a cross-connection control program. Survey data indicated 65 per cent of cross-connections were indirect and 35 per cent were direct. Sudden changes of water demands in piping networks can cause sub-atmospheric pressures within the systems resulting in backflow incidents. Systems with larger variances in ground elevations have the greater possibility of creating sub-atmospheric pressures as water demands in the systems change. Originally published by AwwaRF for its subscribers in 2003